Where does human savagery come from? The animal behaviorist Marc Bekoff, writing in Psychology Today after last month's awful events in Newtown, Conn., echoed a common view: It can't possibly come from nature or evolution. Harsh aggression, he wrote, is "extremely rare" in nonhuman animals, while violence is merely an odd feature of our own species, produced by a few wicked people. If only we could "rewild our hearts," he concluded, we might harness our "inborn goodness and optimism" and thereby return to our "nice, kind, compassionate, empathic" original selves.

If only if it were that simple. Calm and cooperative behavior indeed predominates in most species, but the idea that human aggression is qualitatively different from that of every other species is wrong.

The latest report from the research site that one of us (Jane Goodall) directs in Tanzania gives a quick sense of what a scientist who studies chimpanzees actually sees: "Ferdinand [the alpha male] is rather a brutal ruler, in that he tends to use his teeth rather a lot…a number of the males now have scars on their backs from being nicked or gashed by his canines…The politics in Mitumba [a second chimpanzee community] have also been bad. If we recall that: they all killed alpha-male Vincent when he reappeared injured; then Rudi as his successor probably killed up-and-coming young Ebony to stop him helping his older brother Edgar in challenging him…but to no avail, as Edgar eventually toppled him anyway."

A 2006 paper reviewed evidence from five separate chimpanzee populations in Africa, groups that have all been scientifically monitored for many years. The average "conservatively estimated risk of violent death" was 271 per 100,000 individuals per year. If that seems like a low rate, consider that a chimpanzee's social circle is limited to about 50 friends and close acquaintances. This means that chimpanzees can expect a member of their circle to be murdered once every seven years. Such a rate of violence would be intolerable in human society.

The violence among chimpanzees is impressively humanlike in several ways. Consider primitive human warfare, which has been well documented around the world. Groups of hunter-gatherers who come into contact with militarily superior groups of farmers rapidly abandon war, but where power is more equal, the hostility between societies that speak different languages is almost endless. Under those conditions, hunter-gatherers are remarkably similar to chimpanzees: Killings are mostly carried out by males, the killers tend to act in small gangs attacking vulnerable individuals, and every adult male in the society readily participates. Moreover, with hunter-gatherers as with chimpanzees, the ordinary response to encountering strangers who are vulnerable is to attack them.

Most animals do not exhibit this striking constellation of behaviors, but chimpanzees and humans are not the only species that form coalitions for killing. Other animals that use this strategy to kill their own species include group-living carnivores such as lions, spotted hyenas and wolves. The resulting mortality rate can be high: Among wolves, up to 40% of adults die from attacks by other packs.

Killing among these carnivores shows that ape-sized brains and grasping hands do not account for this unusual violent behavior. Two other features appear to be critical: variable group size and group-held territory. Variable group size means that lone individuals sometimes encounter small, vulnerable parties of neighbors. Having group territory means that by killing neighbors, the group can expand its territory to find extra resources that promote better breeding. In these circumstances, killing makes evolutionary sense—in humans as in chimpanzees and some carnivores.

What makes humans special is not our occasional propensity to kill strangers when we think we can do so safely. Our unique capacity is our skill at engineering peace. Within societies of hunter-gatherers (though only rarely between them), neighboring groups use peacemaking ceremonies to ensure that most of their interactions are friendly. In state-level societies, the state works to maintain a monopoly on violence. Though easily misused in the service of those who govern, the effect is benign when used to quell violence among the governed.

Under everyday conditions, humans are a delightfully peaceful and friendly species. But when tensions mount between groups of ordinary people or in the mind of an unstable individual, emotion can lead to deadly events. There but for the grace of fortune, circumstance and effective social institutions go you and I. Instead of constructing a feel-good fantasy about the innate goodness of most people and all animals, we should strive to better understand ourselves, the good parts along with the bad.—Ms. Goodall has directed the scientific study of chimpanzee behavior at Gombe Stream National Park in Tanzania since 1960. Mr. Wrangham is the Ruth Moore Professor of Biological Anthropology at Harvard University. Mr. Peterson is the author of "Jane Goodall: The Woman Who Redefined Man."